Glands

Glands of the body are classified as either exocrine or endocrine types.
Exocrine glands are glands that retain ducts to body surfaces. During
development, endocrine glands lose their contacts to embryological
surfaces(ducts) and become isolated as small blocks of tissues. Endocrine
glands are therefore referred to as "ductless" glands.

Endocrine and exocrine glands secrete various products. These include
hormones, enzymes, metabolites, and other molecules. In exocrine glands,
products of these cells collect in the duct of the gland and flow toward
the surface to which the duct is in contact. Since endocrine glands
lack ducts, the product is released across the cell membrane into interstitial
spaces around the cells. Diffusion of the product into capillaries
follows.

Most glands of the body are exocrine types with ducts connecting to anatomical
surfaces. Contrast your salivary glands that open into the oral cavity
with sweat glands that deposit their product on the body surface. Both
types of glands are buried in deeper tissues but their products appear on
a superficial surface. Connecting the glands to the surfaces are ducts!

A great deal of variation can be found in the design of glands. They
are classified into simple and compound types. Note there are tubular
and alveolar types!

Secretory cells of exocrine glands release their products into ducts in three
different ways. The mode of secretion can be classified as merocrine,
apocrine, or holocrine.

Cells that secrete products via the merocrine method form membrane-bound
secretory vesicles internal to the cell. These are moved to the apical
surface where the vesicles coalesce with the membrane on the apical surface
to release the product. Most glands release their products in way.

In those glands that release product via the apocrine method, the
apical portions of cells are pinched off and lost during the secretory process.
This results in a secretory product that contains a variety of molecular
components including those of the membrane. Mammary glands release
their products in this manner.

The third type of secretory release, holocrine, involves death of
the cell. The secretory cell is released and as it breaks apart, the
contents of the cell become the secretory product. This mode of secretion
results in the most complex secretory product. Some sweat glands located
in the axillae, pubic areas, and around the areoli of the breasts release
their products in this manner. Sebaceous glands also are of this type.

Regardless of gland type, structural complexity, or mode of secretion,
epithelia are the secretory cells of all glands. Epithelia also
form the ducts that connect the glands to the surface. Remember this
as glandular structures found in tissues can be identified as clusters of
tightly packed cells with very little intercellular space(an epithelial
characteristic). When ducts are present and cut in longitudinal or
cross-section, epithelial cells are also seen making up these structures.
Simple cuboidal epithelia are the most typical type found in the body
and ducts of exocrine glands.

Endocrine glands are the hormone producing structures of the body. Some,
like the thyroid are large and obvious. Others, for instance the islet
cells of the pancreas, are small islands of endocrine cells embedded within
the larger exocrine portion of this organ.

In lacking ducts, endocrine cells release their secretory products into the
interstitial spaces around the cells. The hormones diffuse into nearby
capillaries and are then carried to all parts of the body. Only when
the hormones encounter a "target organ" do they exert an effect.

Notice the islet cells in this view of the
pancreas! A pancreatic duct shown earlier
is visible with some blood vessels and a nerve!